This is an interesting question. (I’ve encrypted my FP1 and being able to mount /data in recovery would be great.)
Android uses dm-crypt which is one of the most used options for full disk encryption on Linux.
I haven’t done this, but I found a few interesting links:
Perhaps that’ll help.
(I don’t think TWRP supports this out of the box. But it my be possible to connect your phone to your PC and mount /data using adb.)